[expr.prim.lambda.closure] Closure types are not required to be non-final

[Halalaluyafail3]

Full name of submitter (unless configured in github; will be published with the issue): Jay Ghiron

Reference (section label): [expr.prim.lambda.closure]/1

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.

Issue description:

It is not stated that the closure type must be non-final. Inheriting from a lambda is frequently used with std::visit to create ad hoc overload sets, and presumably the purpose of specifying non-union classes is to allow for this.

Suggested resolution:

Modify [expr.prim.lambda.closure]/1 as follows:

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union non-final class type, called the closure type, whose properties are described below.

Where the word final is colored as a keyword, as it is in [derivation]/4.